To determine the offsets between the slitlets for Non-FORS data use
Midas
...> OFFSET/MOS
This command will take the calibration spectra of the different slitlets
from
.bdf
and correlate them with the arc spectrum of the first slitlet. Thereby it will
derive their offsets relative to the first slitlet (which will normally
not be in the center of the CCD).